The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio changing speed rate at downshifting for rapid acceleration, such as kickdown and at deceleration of the motor vehicle.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ration control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed rate which is decided by line pressure, pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio.
The pressure applied to the servo device of the drive pulley is increased for upshifting the transmission and decreased for downshifting. Thus, the transmission ratio changing speed rate can be changed by controlling the amount of oil drained from the servo device.
The transmission is generally downshifted at deceleration of the vehicle at braking. In order to prevent the belt from slipping on the pulleys at reacceleration, it is necessary to quickly raise the line pressure while quickly downshifting the transmission to a small pulley ratio stage. To meet such requirements, the transmission ratio control valve is adapted to quickly drain the oil.
On the other hand, the transmission is also downshifted at kickdown. However, if the transmission ratio changing speed rate is as fast as during the above described deceleration, transmission shock occurs, thereby causing unpleasant driving. Therefore, it is preferable to speed up the downshift at deceleration and to slow down the downshift at kickdown.
Japanese Patent Laid Open No. 59-159456 (U.S. Pat. No. 4,565,110) discloses a transmission ratio control system wherein a shift direction changing valve and a shift speed control valve, each having a solenoid operated valve, are provided. Both the valves control the direction of the shifting of the transmission and the shifting speed rate thereof, respectively.
However, in such a system, the valves must be independently provided. Thus, the number of valves increases, resulting in a complicated system. Furthermore, since the shift speed control valve is provided to control the transmission ratio changing speed rate during upshift or downshift, the system cannot control the transmission ratio changing speed rate at kickdown.